This invention pertains to an improved process for hydroformylating an olefinically-unsaturated compound to produce a mixture of aldehyde products.
It is well known in the art that one or more aldehyde products can be produced by contacting under reaction conditions an olefinically-unsaturated compound with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst. One such process, as exemplified in U.S. Pat. Nos. 4,148,830, 4,717,775, and U.S. Pat. No. 4,769,498, involves continuous hydroformylation with recycle of a solution containing the metal-organophosphorus ligand complex catalyst, more preferably, a Group VIII-organophosphorus ligand complex catalyst. Rhodium is a preferred Group VIII metal. Organophosphines and organopolyphosphites are preferred organophosphorus ligands. Aldehydes produced by hydroformylation processes have a wide range of utility, for example, as intermediates for hydrogenation to aliphatic alcohols, for amination to aliphatic amines, for oxidation to aliphatic acids, and for aldol condensation to produce plasticizers.
Until recently, the art disclosed that a high normal to branched (normal/branched or N/I) isomer ratio of the product aldehydes was desirable; however, with present day market demands for specialty chemicals, a low N/I isomer ratio may also be desirable. For the purposes of this invention, a “high” N/I isomer ratio refers to an isomer ratio equal to or greater than 10/1; while a “low” N/I isomer ratio refers to an isomer ratio less than 10/1. Up to the present time, no single catalyst has been capable of producing a wider range of N/I isomer ratios beyond the inherent ratios achievable by the single catalyst itself.
Rhodium-organophosphine ligand complex catalysts, such as rhodium-triphenylphosphine ligand complex catalysts, are known to produce a limited N/I isomer ratio from about 5/1 to about 12/1. To obtain an N/I isomer ratio outside this range, the rhodium-organophosphine ligand complex catalyst must be replaced with another complex catalyst capable of achieving the desired ratio. Converting a manufacturing site from one catalyst to another is costly and non-productive. Additionally, the activity of a rhodium-organophosphine ligand complex catalyst is undesirably low; and thus, the concentration of catalyst including costly rhodium metal required to obtain acceptable process productivity is undesirably high. Moreover, the organophosphine ligand is required to be provided at greater than 200/1 mole-equivalents per mole-equivalent of rhodium.
As an alternative, metal-organopolyphosphite ligand complex catalysts have been shown to provide higher activity and higher N/I isomer ratios in hydroformylation processes, as compared with metal-organophosphine ligand complex catalysts. Higher activity beneficially allows for reduction in the concentration of catalyst, hence expensive metal (e.g., Rh), in hydroformylation reaction fluids. Moreover, the requirement for excess ligand is also reduced. Nevertheless, the N/I isomer ratio obtained from metal-organopolyphosphite ligand complex catalysts is still limited to a narrow range and not sufficiently flexible to meet current day market demands, which range from about 2/1 to 75/1 or higher. More significantly, stabilization of the organopolyphosphite ligand causes concern, because this ligand is hydrolytically unstable, particularly, at low carbon monoxide partial pressures. The art discloses, as shown in U.S. Pat. No. 5,763,679 and WO/2006/020287, that hydrolytic deactivation of metal-organopolyphosphite ligand complex catalysts can be reversed or reduced by conducting the hydroformylation process in a reaction region where the hydroformylation reaction rate is negative or inverse order in carbon monoxide. As used herein, a hydroformylation reaction rate that is “negative or inverse order in carbon monoxide” refers to a region wherein the hydroformylation reaction rate increases as carbon monoxide partial pressure decreases, and wherein the hydroformylation reaction rate decreases as carbon monoxide partial pressure increases.
Typically, the N/I isomer ratio varies inversely with carbon monoxide partial pressure; thus, the ratio decreases as carbon monoxide partial pressure increases, and the ratio increases as carbon monoxide partial pressure decreases. Controlling the N/I isomer ratio via carbon monoxide partial pressure creates problems, however. At low-end carbon monoxide partial pressures (about <15 psi or <103 kPa), hydrogenation of the reactant olefin may increase with efficiency losses to by-product alkanes. At high end carbon monoxide partial pressures (about >50 psi or >345 kPa) overall catalyst activity is reduced and the rate of catalyst deactivation is increased. Thus, the optimal range of carbon monoxide partial pressure, within the negative order regime, imposes restrictions on the N/I isomer ratios achievable.
Prior art, exemplified by U.S. Pat. No. 5,741,945, disclose a hydroformylation process employing a mixture of an organopolyphosphite ligand and a sterically-hindered organophosphorus ligand selected from organophosphine ligands, organomonophosphite ligands, and organomonophosphite-monophosphate ligands. This art illustrates hydroformylation in batch processes wherein an incremental addition of organopolyphosphite ligand may produce a “step-ladder” discontinuity in the N/I product isomer ratio. Moreover, the applicants of the present invention have found that in such processes, the N/I isomer ratio varies over days and weeks and cannot be sufficiently controlled.
Other art, exemplified by WO-A1-2006/098685 and U.S. Pat. No. 5,233,093, teach hydroformylations in batch processes in the presence of mixtures of organophosphine ligands. The references are silent with regard to controlling the N/I isomer ratio over an extended period of time, namely, days or weeks.
WO-A1-2005/120705 discloses a hydroformylation process in the presence of a transition metal and a mixture of a monophosphite ligand and a bisphosphite ligand containing terminal alkoxy groups. The N/I selectivity of the aldehyde product produced is determined by manipulating the mixture of ligands relative to the transition metal. No representation is made of the stability or time dependency of the N/I ratio.
Accordingly, a need exists for a hydroformylation process having improved flexibility over a wider range of N/I isomer ratios as well as improved N/I stability with time. To the extent possible, such a process should not depend upon varying carbon monoxide partial pressure. Moreover, such a process should provide for acceptable catalyst activity, reduce transition metal usage, and reduce undesirable by-product formation.